In certain automatic control systems, reliability and safety are two of the most important factors in gaining acceptance and approval for vital operations. For example, in vital speed control systems for mass and/or rapid transit operations, it is essential to determine the actual speed of a moving vehicle and thereafter to compare the actual speed with the prescribed speed command for a given area or section of the traveled route in order to prevent injury to individuals and damage to equipment. That is, in such vital speed control systems, it is an authoritative requirement that under no circumstance should the actual speed of a moving vehicle exceed the preselected speed command reguest for any given area or section. Hence in order to ensure safe operation, it is mandatory that a failure in the system must not be capable of simulating a higher than the prescribed speed command in a given area or a lower than the actual speed of a moving vehicle. Thus, each subassembly or portion of the system must operate in a fail-safe manner. Further, it is essential that a critical component or circuit failure must not produce spurious oscillations. For example, in amplifiers and oscillators undesirable oscillating signals may occur due to "sneak" feedback paths or large amplitude noise signals may result due to inadvertent increase in the amplitude of voltage furnished by a power supply. The output of a power supply may dramatically increase due to the loss of regulation or due to failure of the filtering network. Thus, every precautionary measure should be taken to envoke good circuit design and layout and to analyze and evaluate each and every failure in order to prevent higher than normal voltage from being produced by a power supply.
Accordingly, it is an object of this invention to provide a novel direct current power supply which operates in a fail-safe manner.
A further object of this invention is to provide a vital type of a regulated reference voltage source the amplitude of which will not be increased by the presence of a critical circuit or component failure.
Another object of this invention is to provide a fail-safe regulated power supply for producing a constant d.c. voltage when and only when no critical circuit or component failure is present.
Yet a further object of this invention is to provide a vital type of a regulated power supply which furnishes a constant d.c. voltage only in the absence of a critical circuit or component failure.
Yet another object of this invention is to provide a fail-safe reference voltage source which utilizes a minimum number of circuit elements.
Still a further object of this invention is to provide a new and improved fail-safe power supply which employs a zener diode, a current-limiting resistor and a four-terminal capacitor for supplying a d.c. reference voltage.
Still another object of this invention is to provide a unique fail-safe source of a direct current reference voltage which is economical in cost, simple in design, reliable in operation, durable in use and efficient in service.
In accordance with the present invention, the fail-safe direct current reference voltage supply includes a coupling means which may be in the form of a transformer having a primary and a secondary winding. The primary winding is connected to a source of alternating current voltage. The upper terminal of the secondary winding of the coupling transformer is connected to the anode electrode of a zener diode. The cathode electrode of the zener diode is connected via a current-limiting resistor to the upper plate of a four-terminal capacitor. The lower plate of the four-terminal capacitor is directly connected to the lower terminal of the secondary winding. A pair of separate output leads are connected to the upper and lower plates of the four-terminal capacitor, and accordingly a constant d.c. reference voltage is developed on the output leads when and only when no critical component or circuit failure is present.
The foregoing objects and other additional features and advantages of this invention will become more fully evident from the foregoing detailed description when considered in conjunction with the accompanying drawings wherein: